1. Field of the Invention
The present invention relates to a network system, and more particularly to SVC (switched virtual circuit: counterpart switched connection) control of ITU-T advice Q.933 or a frame relay exchange complying with a frame relay forum which constitute the network system.
2. Description of the Related Art
In the ITU-T advice Q.933 and the frame relay forum, specific messages are used in the SVC, and a restart (initializing) procedure of cutting a plurality of calls which are currently set or all of calls at once is not defined.
For that reason, in the case where communication is conducted between a plurality of frame relay terminal units (hereinafter referred to as "terminal units") through a frame relay network, when there occurs a fault of a node (a connecting point of a network, for example, a frame relay exchange, etc.), a fault of a terminal unit, a fault of a trunk line that connects between the respective nodes or between the node and the terminal unit, etc., a plurality of calls must be cut. In this case, a process of cutting the respective calls is conducted in accordance with the cutting procedure (normal cutting sequence) defined by ITU-T advice Q.933.
The normal cutting sequence will be now described. For example, in the case where a call is cut between the terminal unit and the frame relay exchange, for example, the frame relay exchange transmits a message "DISC (cut)" to the terminal unit. Upon receiving "DISC" from the frame relay exchange, the terminal unit releases a call between the terminal unit and the frame relay exchange to transmit a message "REL (release)" to the frame relay exchange. Upon receiving "REL" from the other terminal unit, the frame relay exchange releases a call between the frame relay exchange and the terminal unit to transmit a message "REL COMP (release completion)" to the other terminal unit. In this way, the call set between the frame relay exchange and the terminal unit is cut.
However, in the case where a plurality of calls are cut using the normal cutting sequence, there arises a problem stated below. FIG. 25 is an explanatory diagram for explanation of a conventional problem. In the case where communication is conducted under the condition where a plurality of calls (four in FIG. 25) are set between a terminal unit X and a terminal unit Y through a frame relay network as shown in FIG. 25, it is assumed that a fault occurs within the frame relay network. Then, a cutting process is conducted in the frame relay network in accordance with the above-mentioned normal cutting sequence.
In other words, a message "DISC" is transmitted from the frame relay network to the respective terminal units X and Y for every call. Upon receiving the respective messages "DISCs", the respective terminal units X and Y transmit "RELs" corresponding to the respective terminal units X and Y to the frame relay network. Then, the frame relay network receives the respective "RELs" transmitted from the respective terminal units X and Y and transmit "REL COMPs", and transmits the respective "REL COMPs" corresponding to those "RELs" to the respective terminal units X and Y substantially at the same time. Thereafter, upon the reception of the respective "REL COMPs" by the respective terminal units X and Y, all the calls set between the terminal units X and Y are cut.
In the above way, in the normal cutting sequence, the respective messages of "DISC", "REL" and "REL COMP" are exchanged per set call. In other words, in the example shown in FIG. 25, the respective four message exchanges of "DISC", "REL" and "REL COMP" are made between the respective terminal units X, Y and the frame relay network. As a result, during conducting the above-mentioned cutting sequence, a line connecting the terminal unit X and the frame relay network, and a line connecting the terminal unit Y and the frame relay network remarkably rise in load.
For that reason, there was a case in which congestion is generated by execution of the above-mentioned cutting sequence to lower a through-put, resulting in a state where a communication response between the respective terminal units gets remarkably deteriorated.